Hi, I am not an expert but will share some of my experiences. I built 4' x 4' x 6" SFG beds in 2010, in my front yard. I lined the bottoms with a few layers of cardboard and used Mel's mix. Are you planning to use Mel's mix? And what type of soil is under your beds? I had a sandy soil beneath and no problems with drainage. There is Bermuda grass around the area, coming from my neighbors yard. It is a constant battle but it does not come up from underneath the beds, it creeps in on the edges. We have many drought years here in California and a couple of years ago, I let the beds dry out and they are almost impossible to wet again. I now have drought tolerant herbs such as lavender, thyme and sage in those beds and they are doing well. I did have good success with vegetables the first couple of years but had to water consistently.

In 2014, I added raised beds in my back yard. They are larger, 4' x 8' x 18". I did not use Mel's mix. I had a landscape company bring in a suitable mix of soil with some bark fines. I had mushrooms the first year as the bark was decomposing. I add compost and vermicompost and organic fertilizer throughout the year and have had pretty good success with vegetables (not perfect, still have insects and diseases, some things do better than others). I think of it as a modified SFG because I do have square or rectangular areas of specific plants (larger than a square foot though).

I would consider what type of soil you are putting in the beds and what type is below them. A good place to ask for advice on that is the Soil, Compost and Mulch forum.

I would never consider putting any sort of raised bed directly over Bermuda without removal! It is a very persistent, aggressive grass species and will grow through the raised bed soil and you will be dealing with the weeds endlessly. Even the cardboard layer will not deter it permanently!

And don't use landscape fabric and don't add gravel to the bottom of the bed. It is unnecessary for drainage - raised beds will offer fast drainage just by their elevation.